1. Field of the Invention
This invention relates to the field of agriculture, particularly to the area of nutrients, and more particularly to granular nutrients for animals and plants. The invention provides a new method for producing homogeneous mineral granules of animal feed supplements and of plant nutrients and the new nutrient granules prepared by the method.
2. Description of the Related Art
The art of producing granules of material useful in agriculture is an old one. The general technique used in forming granules has consisted of adding sticking materials, usually liquid, to a rolling bed of relatively fine solids. The sticking material agglomerated the solids into a series of shells around a solid nucleus until the granules grew to a preselected size. Unfortunately, the technique suffers from severe limitations relating to a lack of homogeneity in both the composition and size distribution of the granules produced.
The granules using the technology of the prior art frequently vary widely in composition from granule to granule and vary even within the individual granules. The granules run different paths through a rolling granulator during formation because of their different particle sizes, particle shapes, and because of other factors. These different paths and locations of particles within a granulating mass, when the sticking material is added, usually by spraying, cause still further distortions of the granule sizes and compositions.
To obtain a close particle size range of granules for agriculture, such as plant nutrients, it is usually necessary to screen the granules produced, grind the oversize particles and recycle them, along with the undersize particles, to the granulator for further rolling with additional sticking material. The ratio of recycled material to desired size product is usually at least 3 to 1, or higher, for most granular products for agricultural, and is frequently higher than 10 to 1 when a close size range of granules is desired. Until now, it has been particularly difficult to produce small granules which are homogeneous in composition and have a close size-range without high recycle ratios. The production of substantially dust-free granules for agriculture from commodity raw materials has not been possible by any technique taught in the prior art.
In U.S. Pat. No. 4,175,943, P. J. Jordaan and J. P. Deventer provide a method of manufacturing a water soluble mixed NPK fertilizer composition in solid form. Their method first dissolves urea in aqueous phosphoric acid to form urea phosphate adduct solution, adds potassium salts to the adduct while agitating the solid or slurry, and then cools the mixture thus formed to crystallize the solid soluble fertilizer granules. When the mixture is a slurry, the constituents of the mixture are so chosen that the final product contains between 5 and 42% nitrogen, between 2 and 15% phosphorous, and between 1 and 38% potassium. It was also necessary that the pH of a 10% aqueous solution of the mixture be quite acid at between 1.2 and 2.0. Although Jordaan and Van Deventer were able to achieve a homogeneous composition, they unfortunately were required to perform the time consuming and sometimes erratic crystallization operation, and provided no teaching to allow the preparation of homogeneously size granules without screening and substantial recycling of off-sized granules.
M. M. Norton and B. R. Parket in U.S. Pat. No. 4,134,750 provide a method for preparing dust-free ammonium phosphate sulfate and urea-ammonium phosphate sulfate using a common pipe cross reactor. In their method, they produce a high temperature melt by simultaneously feeding and reacting strong ammonia, phosphoric acid, and sulfuric acid in a continuous pipe reactor. The essentially anhydrous melt is discharged into an inclined rotating drum where the melt is cooled and granulated. Drying is achieved by evaporation during the cooling step. Screening and recycle of fines and oversize granules is substantial in the teachings of Norton and Parker.
J. M. Stinson, H. C. Mann, and J. F. McCullough in U.S. Pat. No. 4,217,128 provide further teachings on the production of urea-ammonium polyphosphate from urea phosphates. In their process for high purity area-ammonium polyphosphates, solid urea orthophosphate is melted at a temperature of 260.degree. to 350.degree. F. and the liquid melt is agitated to discharge the carbon dioxide foam formed. The melt is held until the orthophosphate is converted to polyphosphate, which is then discharged into weak aqueous ammonia to produce an aqueous solution of high purity urea ammonium polyphosphate. Although these investigators disclosed mechanical agitation of nutrients, it was don to eliminate carbon dioxide induced foam in a high temperature ammonium phosphate intermediate for the production of liquid fertilizers.
C. B. Kinno, H. Hirayama, and T. Honda in U.S. Pat. No. 4,353,730 provided an improved granulating process wherein granular particulate material is fed to a spouting bed into which is sprayed an adherent and solidifiable liquid together with a gas steam into the spouting bed granulation zone. The priming granules are enlarged by depositing the adherent and solidifiable liquid on the surfaces. The granulation occurs by the growth of a series of pearl-like layers to make large granules from small solid particles. The enlarged granules are withdrawn as product from the last of a series of spouting bed granulation zones. Although these teachings provide for efficient cooling and drying of the granules grown, no method was taught for assuring size and composition homogeneity of the granules produced. The method assures a pearl-like layered structure in the granules, by using the spouting fluid bed.
E. Vogel, in a paper presented at British Sulfur's "Nitrogen 86 " Conference in Amsterdam, The Netherlands, Apr. 20-23, 1986, disclosed a combined drum granulation and fluidized-bed process by which a liquid layer acquired by particles in the bottom of a granulating drum is solidified in a fluidized bed, mounted inside the drum, onto which the particles drop from the drum's lifting flights. This method does not disclose a way to produce granules which are homogeneous with regard to composition and size. As with other granulation methods in the art the granules are layered in structure with most layers at least somewhat different in structure. The particles are different in size because they are grown by the change meeting of granule and liquid in the bottom of the granulating drum.
The prior art literature has taught the layering of liquid on small particles of solids until that particle has grown to the desired size. These teachings lead to granules having varying compositions within the individual granules and between the various granules of the product. Because particle growth only occurs from the change meeting of the small particles with the layering material, granular particle sizes produced vary widely and the only practical way of producing a narrow range of granule particle sizes has heretofore been to use screens and product recycle.
It is an object of this invention to provide a method of producing substantially dust-free, homogeneous, mineral granules which are useful in the field of agriculture.
It is a further object of this invention to provide a method of producing mineral granules which are homogeneous enough in particle size to substantially eliminate the need for product screening and recycle.
It is a further object of this invention to provide new attrition resistant, and substantially dust-free granules of animal feed supplement compounds formed by reacting low cost commodity acids and bases by the method of this invention.
It is a still further object of this invention to provide new attrition resistant granules of plant foods formed by reacting commodity acids and bases by the method of this invention.